[kh99]

You could do this:

<if condition="empty($bbuserinfo[field5])">
<img src="images/header/default_img.png" border="0" alt="$vboptions[bbtitle]" />  
<else/>
<img src="images/header/$bbuserinfo[field5].png" border="0" alt="$vboptions[bbtitle]" />  
</if>

of course you'd want to change default_img to some actual image name. That should give everyone the default unless they have made a choice.

Also, I'm not sure but maybe if you set the field Required to "Yes", new users will have to choose something or go with the default.